Border control and apprehension activity represents a major thrust of the Immigration Reform and Control Act of 1986, but it remains the least investigated aspect of the reform policies. The overall objective of this project is to understand how the economic calculations of Mexican undocumented migrants in the United States are influenced by U.S. policy and by changing economic conditions in Mexico and the U.S. The dynamic effects of the apprehension of undocumented Mexican migrants to the United States will be examined using migration history data from the ENEFNEU survey. Three specific questions will be addressed. First, does apprehending a migrant at any time act as a deterrent to remigration later? Second, how does apprehending a migrant at the border alter current migration behavior? Third, how do relative economic conditions in the U.S. mitigate the effects of apprehension? Hypotheses will be tested using the standard demographic techniques of duration analysis. Specifically, a multiple episode, multiple state competing risk model will be used to estimate the relative effects of apprehension and changing economic conditions in the two countries on the actual and desired length of stay in the United States and on the frequency of migration. Results from the analysis will be used to explore the impact of apprehension policy on the aggregate supply of illegal labor in the United States.